When an oil well has been drilled, or partially drilled, and cased, it is sometimes desired to open a window through the side of the casing to permit drilling of a deviated or side-tracked branch of the well disposed at a substantial angle with respect to the adjacent portion of the original wellbore.
A common way of so opening a window is to set a whipstock in the original wellbore. The whipstock has a generally concave radially inner side, face side, or trough which is angled or tapered downwardly and radially inwardly with respect to an adjacent side of the well wall, typically a casing wall. A starter mill apparatus, including an inwardly and downwardly tapered pilot portion below a milling portion, is lowered relative to the whipstock while also being rotated. The whipstock is furnished with a pilot lug near the top of its concave tapered inner side which engages the tapered pilot portion of the starter mill as it is lowered, directing and forcing the mill into the side wall of the casing facing the whipstock trough to effect penetration of the casing and facilitate cutting of the window. Thus, the starter mill forms a small lateral opening through the casing, or at least mills into the casing sufficiently to form a substantial upwardly facing shoulder in the casing. Then the starter mill apparatus is withdrawn and the milled part of the casing is elongated a window mill, which will form a sufficiently long window in the casing to serve as the upstream end of the side-tracked well branch when drilled therefrom in a manner well known in the art.
A common problem with known apparatus of this type is that the starter mill inevitably mills into the whipstock to a greater or lesser extent as well as the casing, thereby damaging the whipstock, possibly to the extent that it is thereafter impossible to mill a window. This problem results from the fact that it is difficult, if not impossible to determine precisely at what point the starter mill has milled a sufficient amount of the casing to permit the starter mill to be removed from the well and replaced with a window mill in order to complete milling of the window. Normally, milling is commenced with the starter mill and continued for a measurable downward distance which would be sufficient to mill the pilot lug from the whipstock and start the window. When milling with conventional coupled tubulars, such as drill pipe, at exceedingly great depths in a well, it is difficult to accurately measure the relatively short distances required to mill the pilot lug from the whipstock because of the elongation or stretch in the tubulars due to the tubulars' own weight. In the above instance, the degree of accuracy of measurement may range in feet rather than inches depending upon the depth of the operation. The problem of accuracy of measurement is significantly magnified when milling operations are conducted using downhole mud motors suspended from the bottom of coiled tubing strings. The degree of accuracy of measurement under the latter operations, can easily range between ten and twenty feet depending upon the depths of the operations.
The ability to start a window may be further exacerbated by rotational and longitudinal frictional wear to the starter mill pilot and/or the whipstock pilot lug. If the starter mill pilot becomes worn to a smaller outside diameter and/or if the height of the whipstock pilot lug is decreased due to wear as the respective items engage and co-relate with one another to effect the starting of the window, resulting or final dimensions of the respective items may be such, at the point of pilot lug mill-off, that the casing wall is not penetrated or a sufficient shoulder is not created in the casing wall to facilitate subsequent proper milling of the window with the window mill.
This problem is most prevalent in applications involving through-tubing window milling, where a smaller diameter tubular is positioned within the well above a larger size casing in which a window is to be milled. Since the outside diameters of the anchors, whipstocks, mills and other tools employed in milling the casing window must be of a size small enough to pass through the smaller tubular and operate to cause milling of a window in the larger casing, the starter mill must be equipped with milling blades having less depth and pilot noses having smaller diameters as well as whipstocks being furnished with thinner pilot lugs than are used on conventional full bore window milling operations. Thus, excess or slight wear, in some cases, cannot be tolerated without subsequent failure to start the window.
Another problem which exists with known apparatus, again with particular respect to through-tubing applications, stem from the lack of support of the back side (opposite the tapered concave face) of the whipstock by the adjacent casing wall. Since through-tubing whipstocks must be of a much smaller diameter than the casing in which they are set, they must be diagonally angled within the larger casing when set so as to position the top back-side against the inside of the casing opposite the side in which the window is to be milled while the bottom face-side is positioned against the casing adjacent the side in which the window is to be milled. Thus situated in an unsupported position along its full back-side by the casing, the whipstock is susceptible to flexing or bending when contacted by downward forces exerted upon it by the milling tools. Should this occur, flexing of the whipstock would result in the starter mill not being forced into the casing wall by contact of the starter mill pilot nose with the whipstock pilot lug.
With respect to full bore window milling applications, although state of the art whipstocks are equipped with hinges below the tapered concave troughs to enable the back side of the whipstock to lean against and contact the side of the casing opposite the window to be cut at their top ends, the angle between the back-side of the whipstock and the adjacent casing is so small and the unsupported standoff so slight, relative to the depth of the starter mill blades, that slight flexing of the whipstock does not usually effect successful starting of the window. The same is usually true with regard to wear of the window mill pilot nose and the whipstock pilot lug owing to the greater depth of the starter mill blades in full bore window milling operations.